Data acquisition system using delta-sigma analog-to-digital signal converters

ABSTRACT

A delta-sigma A/D converter includes a delta-sigma modulator having an integrating amplifier dispose within the loop of the modulator so that the input of the modulator can be connected to receive relatively low level input currents, and so as to improve DC stability. The modulator is specifically adapted for IC implementation for use in a DAS of a CT scanner so that the current output of a detector of the scanner can be connected directly to the input of the modulator of the converter. The noise shaping characteristics are shaped so as to provide an improved frequency response with minimal spillover. Noise shaping of the converter is provided throughout the dynamic range of the current outputs of CT scanner detectors so as to compensate for the poorer S/N for low input currents. The preferred converter also includes a unique digital filter transfer function, with finite impulse response (FIR) filter characteristics, optimized for CT-DAS applications and having excellent characteristics in the frequency domain and the time domain.

CROSS REFERENCE TO RELATED APPLICATIONS

The following is a continuation application of U.S. patent applicationSer. No. 09/243,708, filed on Feb. 3, 1999 now U.S. Pat. No. 6,252,531issued Jun. 26, 2001, which in turn is a continuation application ofU.S. patent application Ser. No. 08/839,068, filed on Apr. 23, 1997 nowabandoned; which in turn is a continuation application of U.S. patentapplication Ser. No. 08/712,137, filed on Sep. 11, 1996 now abandoned,which in turn is a continuation application of U.S. patent applicationSer. No. 08/326,276, filed on Oct. 20, 1999 now abandoned.

The present application is related to U.S. application Ser. No. 870,059,filed Apr. 17, 1992 in the name of Charles D. Thompson, and issued Dec.28, 1993 as U.S. Pat. No. 5,274,375; and U.S. application Ser. No.870,270, filed Apr. 17, 1992 in the names of Charles D. Thompson,Salvador R. Bernadas, Nicholas R. van Bavel and Eric J. Swanson, andissued Oct. 26, 1993 as U.S. Pat. No. 5,257,026.

FIELD OF THE INVENTION

The present invention relates generally to a data acquisition system(DAS), and more particularly, to a data acquisition system usingoversampled, delta-sigma analog-to-digital (A/D) converters specificallyadapted for use in computed tomography (CT) scanners.

BACKGROUND OF THE INVENTION

Certain signal processing techniques involve the simultaneous detectionof a plurality of analog information signals for the purpose ofacquiring data represented by the signals. For example, certaincommercially available medical imaging systems such as CT scanners areused to image internal features of an object under view by exposing theobject to a preselected amount and type of radiation. Detectors senseradiation from the object and generate analog signals representative ofinternal features of the object.

In the example of CT scanners, those of the third generation typeinclude an X-ray source and X-ray detector system secured respectivelyon diametrically opposite sides of an annular-shaped disk. The latter isrotatably mounted within a gantry support so that during a scan the diskcontinuously rotates about a rotation axis while X-rays pass from thesource through an object positioned within the opening of the disk tothe detector system.

The detector system typically includes an array of detectors disposed asa single row in the shape of an arc of a circle having a center ofcurvature at the point, referred to as the “focal spot,” where theradiation emanates from the X-ray source. The X-ray source and array ofdetectors are all positioned so that the X-ray paths between the sourceand each detector all lie in the same plane, referred to as the “sliceplane” or “scanning plane”, normal to the rotation axis of the disk. TheX-rays that are detected by a single detector at a measuring instantduring a scan is considered a “ray.” Because the ray paths originatefrom substantially a point source and extend at different angles to thedetectors, the ray paths resemble a fan, and thus the term “fan” beam isfrequently used to describe all of the ray paths at any one instant oftime. The ray is partially attenuated by all the mass in its path so asto generate a single intensity measurement as a function of theattenuation, and thus the density of the mass in that path. Projectionviews, i.e., the X-ray intensity measurements, are typically done ateach of a plurality of angular positions of the disk.

In fourth generation CT scanners the detection system comprises acircular array of detectors secured on and at equiangular positionsaround the gantry support, equidistant from the rotation center of thedisk so that the source rotates relative to the detectors. A fan beam isdefined as the ray paths from the rotating source to each detector wherethe point of convergence of each fan beam is the corresponding detector.

The detectors used in CT scanners are usually either of the solid statetype, such as cadmium tungstate detectors each having a scintillationcrystal or layer of ceramic material and a photodiode, or of the gastype, such as Xenon detectors. The X-ray source can provide a continuouswave or a pulsed X-ray beam.

An image reconstructed from data acquired at all of the projectionangles during a scan of both types of machines will be a slice along thescanning plane through the object being scanned. In order to“reconstruct” or “back project” a density image of the section or“slice” of the object in the defined scanning plane, the image istypically reconstructed in a pixel array, wherein each pixel in thearray is attributed a value representative of the attenuation of all ofthe rays that pass through its corresponding position in the scanningplane during a scan. As the source and detectors rotate around theobject, rays penetrate the object from different directions, orprojection angles, passing through different combinations of pixellocations. The density distribution of the object in the slice plane ismathematically generated from these measurements, and the brightnessvalue of each pixel is set to represent that distribution. The result isan array of pixels of differing values which represents a density imageof the slice plane.

While the signals generated by the detectors through the series ofreadings provide the required data to generate the 2-dimensional image,acquiring and processing the data can pose various design problems. Forexample, a large number of detectors must be used for each set ofreadings taken for each projection view, and a large number ofprojection views must be taken during a scan in order to create adetailed image with sufficient resolution (a typical third generation CTscanner contains on the order of 350 to 1000 detectors, with, forexample, 600 to 3000 projection views being taken within a period of 2seconds resulting in data values, i.e., detector readings, on the orderof one million, although these numbers can clearly vary). The resolutionof the image created can be improved by increasing the number ofdetectors used and/or sets of readings, i.e., projection views,utilized. This increases the amount of data acquired and, therefore, theamount of signal information that must be processed. Accordingly, withover approximately one million data values acquired during a typical CTscan the analog signals acquired in each set of readings or views mustbe quickly and efficiently digitized so that computer processing can beutilized to provide relatively fast results.

Thus, in order to process the data received from the array of detectors,a data acquisition system (DAS) is used to process the data throughmultiple channels substantially all at the same time. The DAS includesmeans for converting the plurality of sets of data received from thedetectors as analog signals during each projection view intocorresponding digital signals so that the latter can be processed by adigital signal processor (DSP). However, various problems exist withrespect to current DAS designs. For example, many DASs required for CTscanning require high digitization resolution on the order of onemillion (10⁶) to one or better, i.e., 20 bits or more. While many A/Dconverter techniques are known, some, such as successive approximationA/D conversion, provide inadequate signal resolution and therefore areincapable of achieving a digital signal of 20 bits or more. In thisregard A/D converters using integrators have been designed to providethe required high resolution.

Where DASs are used with a continuous wave X-ray source, any modulationin the X-ray source during a scan over time will create errors. Problemsare also encountered when the DAS is used with a pulse X-ray source. Forexample, artifacts due to variable afterglow readings of X-ray pulsesare not necessarily treated identically for all of the channels. Theseinterpulse values have an overall effect on the values of the detectedanalog signals corresponding to the detected X-rays in response to thepulses of X-rays from the source, and the interpulse values should betaken into consideration to provide accurate readings. In additioncurrent leakage of certain storage devices, disposed in each channel,for storing temporarily stored information can create errors in thesignal conversion.

While some of these problems can be overcome by using a separate A/Dconverter for each channel, until recently such an approach has beenimpractical because of its prohibitive cost. With the dynamic range ofthe analog signals provided in each channel on the order of 10⁶ to 1, alinear ramp A/D converter is also impractical. One DAS which overcomesor at least minimizes many of the above problems is described in U.S.Pat. No. 5,138,552, patented in the name of Hans J. Weedon and EnricoDolazza, issued Aug. 11, 1992 and assigned to the present assignee (the“Weedon et al. Patent). The latter patent describes a DAS usingnon-linear digitization intervals by employing a non-linear ramp A/Dconverter.

In addition, CT scanners use detectors providing low level outputcurrents. In general solid state detectors each include a layer ofscintillator crystal or ceramic material for generating low energyphotons as a function of high energy photons received from the X-raysource. A photodiode is provided with each scintillator crystal forgenerating a current as a function of the low energy photons emitted byand detected from the corresponding scintillator crystal or ceramicmaterial. Since the photodiodes provide a low level current, apreamplifier, in the form of a transimpedance amplifier, is typicallyprovided to convert the current to a voltage at an appropriate level sothat it can be converted to a digital signal. In fact in some CTscanners using gas detectors, similar transimpedance amplifiers areutilized for the same reasons. Analog filtering of the output of eachtransimpedance preamplifier, prior to A/D conversion, is carried out tosuppress the out-of-band portions of the wideband noise originatingwithin the preamplifiers and the photodiodes that precede them. Doingthis filtering before A/D conversion reduces the noise generating analias within the band of frequencies containing the information dataduring A/D conversion. Customarily, sample and hold circuitry isprovided before each A/D converter to hold each successive samplethroughout the time period needed to complete the A/D conversion.

In certain CT scanners the preamplifiers and filters are apportionedamong subgroups of detectors and filters and the analog signal outputsof each subgroup of detectors and filters are analog multiplexed priorto being converted to a digital signal. But analog multiplexing createsdifficulties in matching the conversion characteristics of the A/Dconverters for the various subgroups owing to the need for a very highnumber of bits of resolution in converter output signals in order toreconstruct the image. Differences in conversion characteristics cancause noticeable “banding artifacts” in the final image. The bandingartifacts appear as intensity variations with the reconstructed imagewith considerable lower spatial frequency so that they are usuallynoticeable. Selecting photodiodes which are physically spaced from oneanother for each group can reduce these artifacts. However, this willincrease the likelihood of high spatial frequency components of theseartifacts appearing in the image. These high spatial frequencycomponents may also be filtered with a low pass filter, if desired, witha loss of only of some high-spatial frequency detail in the final image.

Further, spaced apart detectors for each subgroup leads to complexinterconnections complicating data transfer. In addition spaced apartdetectors using a time multiplexed architecture increases the physicaldistance between some of the detectors and the respective preamplifiers,increasing the chances of pick-up of extraneous electrical signals asnoise.

A DAS having high resolution that has been developed for CT scannersthat overcomes or at least reduces the effects of these problems is aDAS using delta-sigma oversampled A/D converters, described in U.S. Pat.No. 5,142,286 issued in the names of David B. Ribner and Michael A. Wufor Read-out Photodiodes Using Delta-sigma Oversampled Analog-to-DigitalConverters (the Ribner et al. Patent). The Ribner et al. Patentdescribes a high-resolution A/D signal converter using componentscommonly used to process audio signals for use in processing data from aCT scanner. Conversion is provided through the use of oversampled,interpolative (or delta-sigma) modulation followed by digital low-passfiltering, typically using an finite impulse response (FIR) filter, andthen by decimation. “Oversampling” refers to operation of the modulatorat a sampling rate many times above the signal Nyquist rate, whereas“decimation” refers to subsampling so as to reduce the sample rate tothe Nyquist rate. The ratio R of the oversampling rate to the signalNyquist rate is designated the “oversampling ratio”. As described in theRibner et al. Patent, delta-sigma A/D converters having single-bitquantizers in the overall feedback loops of their delta-sigma modulatorscan have very linear predictable conversion characteristics so thatmatching the conversion characteristics of a plurality of delta-sigmaA/D converters can be easily accomplished by so designing them in thesame way. This result makes it feasible to use such a converter witheach photodiode and preamplifier combination of a CT scanner, withoutthe need to time multiplex in the analog domain.

The design proposed in the Ribner et al. Patent, necessarily requires aseparate transimpedance preamplifier for generating an analog outputsignal responsive to the photocurrent of the corresponding photodiode ofthe solid state detector. The analog output signal is accompanied bywideband noise. Each analog output signal is applied to an analog,anti-alias, lowpass filter, whose output is provided to the input of acorresponding delta-sigma A/D modulator of an A/D converter. Theconverter includes a data rate decimator and digital filter whichsuppresses quantization noise from the delta-sigma modulator portion ofthe A/D converter, as well as a component arising from remnant widebandnoise from the preamplifier.

While the use of delta-sigma modulators for A/D conversion in a DAS, asproposed in the Ribner et al. Patent, provides certain advantages overthe prior art A/D converters using integrators, the design proposed bythe patentees has certain drawbacks. For example, there is currentlygreat interest in decreasing the overall cost of CT scanners. The DASsignificantly contributes to that cost. While the oversamplingdelta-sigma modulator and data rate decimator and digital filter as anA/D converter easily lend themselves to integration fabricationtechniques, the transimpedance pre-amplifier and anti-alias low-passfilter do not. Currently, such analog circuitry would be expensive tofabricate as a part of an integrated chip set including the delta-sigmamodulator, probably more expensive than using discrete components basedupon current integration techniques. Providing a separate transimpedancepreamplifier and analog filter for each detector in discrete form as thefront end of each channel of a DAS, nevertheless adds significant costto the DAS where, for example, the number of channels needed are on theorder of 350 to 1000 channels. It is desirable therefore to simplify thefront end of the DAS so that it can be made entirely as integratedcircuitry so as to reduce the cost of the DAS.

In addition a design tradeoff exists between a DAS having a spectralresponse optimized for the frequency domain and a DAS optimized for thetime domain. More particularly, when sequentially reading the output ofa channel, it is clear that the reading during each sampling intervalshould be as independent as possible from the previous readings takenfrom that channel, as well as readings taken from the other channels.This effects the time domain properties of the A/D converter. Any“spill-over” or “cross talk” of a signal in the channel from a priorsampling interval (sometimes referred to as “view-to-view cross talk”),thus will have a negative effect on the time domain properties of theconverter. On the other hand, the frequency response of the converterlargely determines the signal-to-noise ratio (S/N) and thus the qualityof the signal processed through the channel. The DAS using a low passanalog filter, such as a best estimate filter of the type described inU.S. Pat. No. 4,547,893 issued Oct. 15, 1985 to Bernard M. Gordon andassigned to the present assignee, for shaping the spectral response ofthe output of the transimpedance amplifier is optimized for itsfrequency domain characteristics, at the cost of some of its time-domainproperties, with spillover of as much as 25% not being unusual (i.e.,25% of the signal is from previous readings in the channel). On theother hand, an integrator type analog filter, is optimized for itstime-domain characteristics, since the integrator is cleared or nulledafter each sampling interval, before the next sampling interval. Thisinsures little or no spillover, however, at the cost of negativelyimpacting its frequency-domain properties with a substantial amount ofhigh frequency noise being present.

In addition to the foregoing, electronic noise can be a significantproblem in DASs used for CT scanners, particularly at low level detectorsignal levels. The design described in the Ribner et al. Patent uses adelta-sigma modulator and FIR digital filter. The noise levels of thedesign tend to remain substantially the same throughout the dynamicrange of the input signal. Further, the noise level of X-ray flux is notsubstantially constant for all levels of flux, but instead isapproximately proportional to the square root of the number of photonspresent. Thus, where the electronic noise of the circuit issubstantially at a relatively fixed level, the S/N level of the analogsignal provided in each information transmission channel (which is afunction of both the noise level of the X-ray flux and the electronicnoise), prior to digitization, tends to worsen as the signal getssmaller.

Finally, DC stability is of great concern in CT scanner DAS applicationssince drift of the detected signal can seriously effect the uniformityof channel to channel performance, and thus the quality of the imagereconstructed from the data derived from the acquired signals.

OBJECTS OF THE INVENTION

It is therefore a general object of the present invention to reduce thecosts of a DAS of the type using delta-sigma A/D conversion.

In is another object of the present invention to provide an improved DASdesign of the type using delta-sigma A/D conversion and adapted to beeasily implemented in integrated circuit form and responsive to lowamplitude current inputs received from CT scanner detectors.

Another object of the present invention is to provide an improved DAS ofthe type using delta-sigma A/D converters without the need for analogsignal time multiplexing.

And another object of the present invention is to provide an improvedDAS of the type using delta-sigma converters and having an improvedfrequency response with nominal spillover.

Yet another object of the present invention is to provide an improvedDAS of the type designed to provide unique noise shaping of thedelta-sigma A/D converters throughout the dynamic range of the currentoutputs of CT scanner detectors so as to compensate for the poorer S/Nfor low X-ray photon levels.

Still another object of the present invention is to provide a DAS of thetype using delta-sigma A/D converters having a unique digital filtertransfer function, with finite impulse response (FIR) filtercharacteristics, optimized for CT-DAS applications and having excellentcharacteristics in the frequency domain and the time domain.

And yet another object of the present invention is to provide a bestestimating, optimum, transient response filter which is an improvementover the filter design described in U.S. Pat. No. 4,547,893 issued toGordon.

And still another object of the present invention is to provide animproved delta-sigma A/D converter for use in a DAS of a CT scannerhaving improved DC stability.

SUMMARY OF THE INVENTION

The invention relates to a delta-sigma A/D converter including adelta-sigma modulator having an integrating amplifier dispose within theloop of the modulator so that the input of the modulator can beconnected to receive relatively low level input currents, and so as toimprove DC stability. The modulator is specifically adapted for ICimplementation for use in a DAS of a CT scanner so that the currentoutput of a detector of the scanner can be connected directly to theinput of the modulator of the converter. The noise shapingcharacteristics are shaped so as to provide an improved frequencyresponse with minimal spillover. Noise shaping of the converter isprovided throughout the dynamic range of the current outputs of CTscanner detectors so as to compensate partially for the poorer S/N forlow input currents. The preferred converter also includes a uniquedigital filter transfer function, with finite impulse response (FIR)filter characteristics, optimized for CT-DAS applications and havingexcellent characteristics in the frequency domain and the time domain.

Other objects of the present invention will in part be obvious and willin part appear hereinafter. The invention accordingly comprises theapparatus possessing the construction, combination of elements andarrangement of parts exemplified in the following detailed disclosureand the scope of the application of which will be indicated in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the presentinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a diagram, partially in schematic and partially in block form,of a computerized tomography scanner of the third generation type, whichcan be designed to include the present invention;

FIG. 2 is a block diagram of one channel of a prior art DAS of typeemploying time multiplexing, an anti-alias low-pass analog filter, andauto-ranging (or floating point amplifier) A/D conversion techniques;

FIG. 3 is a block diagram of one channel of a prior art DAS of typeemploying time multiplexing, an analog integrate and dump (or “boxcar”)filter, and auto-ranging (or floating point amplifier) A/D conversiontechniques;

FIG. 4 is a block diagram of one channel of the prior art DAS of thetype described in the Ribner et al. Patent;

FIG. 5 is a block diagram of one channel of a the DAS designed inaccordance with the present invention;

FIG. 6 is a block diagram of the preferred embodiment of delta-sigma A/Dconverter shown in FIG. 5;

FIG. 7 is a block diagram of the preferred implementation of thedelta-sigma modulator shown in FIG. 6;

FIG. 8 is a graphical illustration of a comparison of two low passanalog filters, an integrating type filter and the spectral response ofthe digital FIR filter utilized in the preferred embodiment of thepresent invention shown in FIG. 6; and

FIG. 9 is a graphical illustration showing a reduction of the electronicnoise for one channel of the preferred DAS employing the delta-sigma A/Dconverter as the input signals get smaller.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the essential elements of a CT scanner of the thirdgeneration type. The CT scanner 10 comprises a gantry including a gantrysupport frame 12 for rotatably supporting an annular disk 14 about arotation axis indicated at 16. The disk 14 has an opening 18 forreceiving the object 20 to be scanned, the object typically beingsupported on a pallet or cantilevered table, indicated at 22. The disk14 supports an X-ray source 24 and detector array 26 on diametricallyopposite sides of the opening 18. As seen the fan beam 28 created by thesource 24 is directed toward the detector array 26. The detector arrayis connected to a DAS 30 for processing the data received from thedetector array 26. The data is processed by DAS 30 and stored in memory32. A back projection computer 34 is adapted to process the data in amanner which is well known using Radon mathematics so that the data canbe provided to a display processor 36 for archival storage as indicatedat 38, for providing a hard copy as indicated at 40, or for displayingon a console as indicated at 42.

One type of prior art DAS commonly used in a CT scanner of the typeusing solid state detectors is illustrated in FIG. 2. In typical CTscanners there are from about 350 to 1000 detectors (only one beingshown in FIG. 2) producing an equal number of signals through an equalnumber of channels. As shown, a solid state detector 50 comprises (1) ascintillator crystal or ceramic 52 for emitting low energy photons inresponse to and as a function of the high energy photons emitted by theX-ray source of the CT scanner and detected by the crystal, and (2) aphotodiode 54 for providing a current output as a function of and inresponse to the low energy photons detected from the crystal or ceramicmaterial. Thus, the detector 50 provides a current output signal as afunction of and responsive to the X-ray photons detected. The diode 54is connected to a null-point transimpedance pre-amplifier 56 forconverting the current output of the diode to a voltage at anappropriate level. The output of the pre-amplifier 56 is connected tothe anti-aliasing, aliasing, low pass analog filter 58. The channelsdefined by the respective detectors 50, pre-amplifiers 56 and filter 58,through which the corresponding analog signals are transmitted, aredivided into groups, with the channels of each group time sharing an A/Dconverter so as to reduce the overall cost of the DAS. The analogsignals associated with each group are applied to the common A/Dconverter 62 through a time analog signal multiplexer 60 in a sequentialmanner so that all of the analog signals transmitted through thechannels of a group can be independently converted by the common A/Dconverter 62. The converter is preferably of the type described as afloating point (or auto-ranging) A/D converter, which comprises both afloating point amplifier and an A/D converter and operates in a mannerwell known in the art. An example, of an auto- ranging A/D converter isdescribed in U.S. Pat. No. 5,053,770 issued Oct. 1, 1991 in the names ofEliot Mayer, Louis R. Poulo, Jeffrey L. Sauer and Hans J. Weedon. Onceconverted, the signals are applied to a digital data multiplexer 64.

As shown in FIG. 3, an analog integrate and dump (or boxcar) filter 70can be substituted in place of filter 58. Again a multiplexingarrangement is provided in order to reduce the number of A/D convertersneeded for processing the signals provided from all of the detectors.

Using the analog signal time multiplex arrangement shown in FIG. 2 orFIG. 3, the signal conversion frequently is not identical for thesignals processed through the different groups to the degree necessaryto achieve the desired high resolution for a relatively large dynamicrange. The variabilities among commonly used A/D converters can thusresult in nonuniform readings. As described above, the low pass, analogfilters 58 can be designed to provide a decent frequency response, butexhibit poor time domain response. For example, in FIG. 8 the frequencyresponse of a low pass, analog two pole filter having a rolloff of −12dB/octave beginning at the cut off frequency of 260 Hz is shown at A,while the frequency response of low pass, analog three pole filterhaving a rolloff of −18 dB/octave beginning at the cut off frequency of380 Hz is shown at B. However, spillover of as much as 25% occurs withthe use of these filters. Use of the analog integrate and dump or boxcarfilter 70, as shown in FIG. 3, provides a better time domain response,e.g., a spillover on the order of less than 1%. But as shown in FIG. 8,curve C, the improvement in the time domain is at the expense of apoorer frequency response, where the effective rolloff of curve C asseen in FIG. 8 is much smaller than that achieved by the analog low passfilters represented by curves A and B.

The delta-sigma A/D converter design, described in the Ribner et al.Patent, provides an improvement over the DASs of the type described inconnection with FIGS. 2 and 3, using analog, low pass filters and boxcarfilters. A simplified block diagram of the delta-sigma A/D converterdesign is shown in block diagram form in FIG. 4. As shown the photodiode54 of each detector 50 is connected to a null-point transimpedancepre-amplifier 56, which in turn is connected to an anti-aliasinglow-pass analog filter 58, the identical arrangement provided in theFIG. 2 system. However, the output of the filter 58 is connected to adelta-sigma A/D converter 80 comprising the oversampling, interpolative,delta-sigma modulator 82 and the data rate decimator and digital filter84. Each channel is therefore defined by a detector 50, transimpedanceamplifier 56, filter 58, modulator 82 and decimator filter 84,eliminating the need for an analog multiplexer since each channel hasits own A/D conversion. In addition, the delta-sigma modulator 82 isdesigned to push much of the noise (dominated by quantization noise)into the higher frequencies and subsequently removed by the data ratedecimator and digital filter 84. Each channel is connected to a digitaldata sequencer 86 for providing a sequencing of data to the interfacewith the storage device, and back projection computer shown in FIG. 1.While the arrangement provides a time sharing digital data sequencer 86,it should be evident that time sharing in the digital domain, wheresignals are either of one of two values is far less likely to lead toerrors, than an analog signal time multiplexing system requiring thetime sharing transmission of signals which are being measured within adynamic range of approximately one million to one, where a quantizationshift of analog signal (equal to one millionth of the full signal value)can introduce an error. In addition, the arrangement of FIG. 4 providesan improvement over the arrangements described with respect to FIGS. 2and 3, since each delta-sigma converter 80, comprising the oversamplingdelta-sigma modulator 82 and the data rate decimator and digital filter84, has single-bit quantizers in the overall feedback loops of itsdelta-sigma modulator 82 and therefore has very linear conversioncharacteristics. Matching the conversion characteristics of a pluralityof delta-sigma A/D converters can be easily done by designing them tohave the same single-bit quantizers in the overall feedback loops oftheir delta-sigma modulators. This result makes it feasible to use sucha converter with each photodiode 54 and preamplifier 56 combination,without the need to time multiplex in the analog domain.

In addition, as described in the Ribner et al. Patent, two major factorsdetermine the resolution of the delta-sigma A/D converters. One factoris the overall sampling ratio R, and the other factor is the “order” ofthe modulator. It is preferable to use a higher-order modulator in a CTscanner since the oversampling ratio R need not be quite so large; giventhat there are hardware limitations on how short the duration of asampling interval can be made. Reducing the number of samples requiredfor obtaining a specified bit resolution from the delta-sigma A/Dconversion will reduce the time required to acquire the data of eachprojection view. However, “order” indicates the relative degree ofspectral shaping that is provided by the delta-sigma modulator. Higherfrequency selectivity is obtainable with a higher order modulator at theexpense of increased hardware complexity, particularly in the decimationfilter required to suppress quantization noise from the modulator. Asdescribed in the Ribner et al. Patent, one FIR digital filter designthat is suited for use in the decimation filter of a delta-sigmamodulator to form a frequency selectivity against quantization noise hasa frequency response of

sinc^((L+1))(ωT),  (1)

wherein L is the order of the delta-sigma modulator,

ω is the radian frequency, and

T is the modulator period.

It is submitted, however, that the design described in the Ribner et al.Patent has its shortcomings. First, the use of the null-pointtransimpedance amplifier 56 and the analog filter 58 makes the designdifficult to implement entirely in IC form. While the converter 80 andsequencer can be implemented in IC form, the front end comprising thepre-amplifier 56 and filter 58 cannot, due to the nature of thosecomponents. Thus, a DAS comprising 350 and 1000 channels requires 350 to1000 front end discrete analog components to form the pre-amplifier andfilter. This adds considerable costs to the DAS. In addition, typicaldelta-sigma modulators and decimators and filters were originallydesigned for processing audio signals where DC stability is of littleconcern and the S/N of the signal remains substantially the same throughthe dynamic range of the audio signal. However, as previously describedDC stability is very important in processing data signals in a CTscanner, where DC drift can cause inaccuracies in the reconstructedimage produced from the data. Further, while the S/N ratio of audiosignals tends to remain the same throughout the dynamic range of thesignal, the noise level of X-ray flux is not substantially constant forall levels of flux, but instead is approximately proportional to thesquare root of the number of photons present. Thus, where the electronicnoise of the circuit is substantially at a relatively fixed level, theS/N level of the analog signal provided in each information transmissionchannel, prior to digitization, tends to worsen as the signal getssmaller.

In accordance with the present invention the delta-sigma A/D converteris provided specifically for use in a DAS of a CT scanner, havinggreater DC stability, improved performance in both the frequency andtime domain, and adapted to be implemented in IC form so as to reducethe cost of the DAS. As shown in FIG. 5, the current output of the diode54 is applied directly to the input of the current input, oversamplingdelta-sigma modulator 90 of the delta-sigma A/D converter 94. The outputof the modulator is applied to the data rate decimator and digitalfilter 92, for reducing the data rate to within the bandwidth ofinterest and for filtering out high frequency noise. The output of thefilter 92 is applied to the digital data sequencer 96. As will be moreevident hereinafter, by making the modulator 90 a current input devicethe design allows for the elimination of the input transimpedancepre-amplifier, designated 56 in FIG. 4. In addition, the anti-aliasfilter 58 of FIG. 4 can be eliminated as well by designing the noiseshaping characteristics of the modulator 90 and the data rate decimatorand digital filter 92 to provide improved results. Thus, the componentsthat were heretofore required to be made as discrete components areeliminated.

More particularly, referring to FIG. 6 the current input from thedetector, represented as a current source 100 is applied directly to theinput of the modulator 94. Specifically, the current is applied to theinverting input of an input integrating amplifier 102, the latter havinga non-inverting input connected to ground, and an impedance 104connected in the feedback path between the output and inverting input.The impedance is preferably in the form of a switching capacitor, whichprovides an impedance as a function of the capacitance value of thecapacitor and the frequency at which it is switched. The output of theintegrating amplifier 102 is applied to a noise shaping circuit 106,preferably comprising a plurality of integrators as described inconnection with FIG. 7, and having a transfer function H(z). The outputof the noise-shaping circuit 106 is connected to an input of an A/Dconverter 108 which digitizes the analog input at a sampling ratedesignated f₁. The output of the converter 108 is applied to the inputof a quantizer in the form of a digital to analog (D/A) converter 110,which in turn applies its output signal to the non-inverting input ofthe integrating amplifier 102 so as to form a feedback loop, as isrequired of a delta-sigma modulator. The output of the A/D converter 108is also applied to the input of the data rate decimator and digitalfilter 112 which reduces the sample rate to f₂. The digital filter ispreferably a finite impulse response (FIR) filter having predeterminedcoefficients stored in read only memory (ROM) 114 for setting the tapsof the filter.

In CT scanners of the third generation type, the bandwidth of interestis typically between DC (0 Hz) and f_(INF), where f_(INF) typically isbetween about 100 Hz and 500 Hz. The preferred sampling ratio, R, is setat 128, with the Nyquist rate set to be twice the highest frequency ofinterest (e.g., 500 Hz). Thus, f₁ is preferably set at about 128 KHz,while the decimation rate is set to reduce the data rate output of A/Dconverter 108 to about 1 KHz (f₂). It should be appreciated wheref_(INF) is higher, as is the case with some fourth generation CTscanners (where f_(INF) can be as high as 10 kHz), the frequencies canbe adjusted to provide the desired result.

The specific implementation of the modulator 94 is shown and describedin related application, U.S. application Ser. No. 870,059, filed Apr.17, 1992 in the name of Charles D. Thompson, and issued Dec. 28, 1993 asU.S. Pat. No. 5,274,375 (the “'375 Patent”). The specific implementationdescribed in the '375 Patent, was derived from the present invention. Asdescribed, and reproduced in FIG. 7, an input current signal is providedto the positive input of summing junction 120, the output of which isapplied to the input of the first integrator stage 122. The firstintegrator stage 122 is comparable to the integrating amplifier 102 ofFIG. 6. The integrator stage preferably has a gain coefficient of one.The output of integrator stage 122 is applied to the positive input of asecond summing junction 124, the output of which is connected to asecond integrator stage 126. This second integrator stage 126 alsopreferably has a gain coefficient of one. The output of the secondintegrator stage 126 is applied to the input of a third integrator stage128, the latter preferably having a gain coefficient of 0.2. The outputof the third integrator stage 128 is applied to the input of a summingjunction 130. The output of the latter is applied to the input of afourth integrator stage 132 having a coefficient preferably set for 0.2,with the output of the stage 132 being applied to the input of a fifthintegrator stage 134. The stage 134 preferably has its gain coefficientset at 0.2. A feedback path, indicated at 136 is provided between theoutput of the stage 128 and the summing junction 124. A similar feedbackpath, indicated at 138, is provided between the output of the stage 134and the summing junction 130. As indicated the feedback coefficient ofpath 136 is preferably set at 0.0115, while the feedback coefficient ofpath 138 is preferably set at 0.020. The output of each of theintegrator stages 122, 126, 128, 132 and 134 are applied to positiveinputs of a summing junction 140 which forms a part of the feedback pathof the modulator. The output of each integrator stage 122, 126, 128, 132and 134 is modified by an attenuation coefficient of 0.95, 0.45, 0.60,0.45 and 0.15, respectively before being applied to the summingjunction, as indicated at 142, 144, 146, 148 and 150. It will beappreciated that as shown, the configuration provided by the stages 126,128, 132 and 134, and their interconnections and connections to summingjunction 140 provide the noise shaping transfer function H(z) referredto at 106 in FIG. 6. Further, while five amplifiers are used to providea fifth order modulator, other orders can be designed to provideddifferent degrees of resolution. The greater the order of the modulatorthe better the resolution, but the greater the complexity. It isbelieved that for satisfactory results at least a second order modulatormust be utilized with a fifth order modulator being preferred.

In FIG. 7, the output of summing junction 140 is applied to the input ofa tri-level quantizer 152 comprising two comparators 154 and 156, theoutputs of which are connected to the filter 112 (seen in FIG. 6) and toa three level D/A converter (DAC) 158. This three-level D/A converter isoperated with normalized DAC feedback levels of −1 and +1 in addition tothe third “do nothing” level. The analog full scale input range to thedelta-sigma modulator is then normalized to between +1 and −1. Thesenormalized input and feedback levels are used in the simulationsdescribed in the '375 Patent.

Referring to FIG. 9, there is a graphical representation of theelectronic noise of the modulator 94 as a function of input signallevel, the latter ranging from full positive scale to full negativescale, with the relative noise performance being plotted on the verticalaxis. This graph illustrates the condition when the quantizer thresholdsfor the comparators are optimized for electronic noise. As shown theelectronic noise is inversely related to the level of input signal. As aresult, because of the 4 bB drop in electronic noise at 0.0 inputcompared to full scale, the overall S/N ratio of the data signal at lowinput levels will be improved, which helps compensates for the poorerS/N ratio associated with the relatively small number of photons andtheir associated noise at low level X-rays, which as previouslydescribed is approximately proportional to the square root of the numberof photons present.

In addition, the data rate decimator and digital filter 112 of thedelta-sigma A/D converter shown in FIG. 6 includes an improved FIRfilter designed so as to achieve greater roll off than achieved by theprior art anti-alias, low pass filters, and the digital filter used inthe design of the Ribner et al. Patent so as to have improved frequencyresponse characteristics. In particular, a standard FIR filter is usedand is designed with 384 taps. The ROM 114 is programmed with 384coefficients predetermined to provide the desired frequency response.The coefficients are identified in Appendix A, attached hereto andforming a part of this disclosure. The FIR filter should be designed toprovide a gentle roll-off at low frequencies from DC to the maximuminformation frequency of interest, I_(INF) (which for third generationCT scanners can range from 100 Hz to 500 Hz), so as to include thebandwidth of interest, with some substantial attenuation (e.g., −20 dB)at the scanner view rate (i.e., the delta-sigma output sampling rate,f₂), and a continuously increasing roll-off rate up to about twice theview rate (2*f_(VR)) where the roll-off exceeds −100 dB/octave and thenominal magnitude response is a maximum of −126 dB; beyond thisfrequency, the magnitude response must remain below −126 dB. Theimprovement in the frequency response is, however, attained even withexcellent time domain performance (maximum spillover of about 4%, asdefined by the FIR filter coefficients).

The foregoing delta-sigma A/D converter is thus an improvement over theprior art converters described. The current input, oversampled,delta-sigma A/D converter is responsive to the low level current inputsreceived from detectors of a CT scanner and can be completelyimplemented in IC form thereby reducing the overall costs of a CTscanner DAS. A plurality of delta-sigma A/D converters can be used toprocess a corresponding number of signals received from the detectorarray of a CT scanner, without the need for analog signal multiplexing.The DAS using delta-sigma A/D converters designed in accordance with thepresent invention has an improved frequency response with nominalspillover. The unique noise shaping of the delta-sigma A/D convertersthroughout the dynamic range of the current outputs of CT scannerdetectors compensates for the poorer S/N associated with photondetection. The delta-sigma A/D converter is provided with a uniquedigital filter transfer function, with finite impulse response (FIR)filter characteristics, optimized for CT-DAS applications and havingexcellent characteristics in the frequency domain and the time domain.The delta-sigma A/D converter of the present invention provides a bestestimating, optimum, transient response filter which is an improvementover the filter design described in U.S. Pat. No. 4,547,893 issued toGordon. Finally, by having the front end of each channel disposed withina control loop, the improved delta-sigma A/D converter provides improvedDC stability for use in a DAS of a CT scanner.

Other modifications and implementations will occur to those skilled inthe art without departing from the spirit and the scope of the inventionas claimed. Accordingly, the above description is not intended to limitthe invention except as indicated in the following claims.

What is claimed is:
 1. An improved oversampling, delta-sigma A/Dconverter comprising: converter input means for receiving an analoginformation input current, and converter output means for providing adigital output signal at a predetermined word rate; a delta-sigmamodulator for receiving an analog information signal as a function ofsaid analog information input current, said modulator comprising: (a)modulator input means coupled to said converter input means, (b)modulator output means for providing an intermediate digital signalrepresentative of said analog information input current, (c) an inputintegrating amplifier for generating an integrated output signal as afunction of the input current, (d) means for noise shaping theintegrated output signal so as to provide an analog noise shaped signal;(e) means for converting the analog noise shaped signal to saidintermediate digital signal as a function of the analog filtered signalat a sampling rate of f₁; and (f) feedback digital-to-analog signalconverter means for generating an analog feedback signal as a functionof said intermediate digital signal and applying said analog feedbacksignal to said modulator input means; and means for decimating the datarate of and for filtering said intermediate digital signal so as togenerate said digital output signal at a word rate f₂.
 2. A converteraccording to claim 1, wherein said means for noise shaping theintegrated output signal includes (i) at least one additionalintegrating amplifier connected between the output of said inputintegrating amplifier and the modulator output means, for providing asecond integrated output signal, (ii) means for weighting the integratedoutput signals of said integrating amplifiers so as to generatecorresponding weighted integrated output signals, (iii) means forsumming said weighted integrated output signals so as to generate asummed signal, and (iv) means for generating said intermediate digitalsignal as a function of said summed signal.
 3. A converter according toclaim 1, wherein said modulator is a fifth order modulator.
 4. A CTscanner comprising a source of X-rays, an array of detectors fordetecting X-rays emitted by said source and received by said detectors,and a data acquisition system for processing signals generated by saiddetectors, said data acquisition system comprising a plurality ofoversampling delta-sigma A/D converters, each of said delta-sigmaconverters comprising: converter input means for receiving an analoginformation input current, and converter output means for providing adigital output signal at a predetermined word rate; a delta-sigmamodulator for receiving an analog information signal as a function ofsaid analog information input current, said modulator comprising (a)modulator input means coupled to said converter input means, (b)modulator output means for providing an intermediate digital signalrepresentative of said analog information input current, (c) an inputintegrating amplifier for generating an integrated output signal as afunction of the input current, (d) means for noise shaping theintegrated output signal so as to provide an analog noise shaped signal;(e) means for converting the analog noise shaped signal to saidintermediate digital signal as a function of the analog filtered signalat a sampling rate of f₁; and (f) feedback digital-to-analog convertermeans for generating an analog feedback signal as a function of saidintermediate digital signal and applying said analog feedback signal tosaid modulator input means; and means for decimating the data rate ofand for filtering said intermediate digital signal so as to generatesaid digital output signal at a word rate f₂.
 5. A scanner according toclaim 4, wherein said means for noise shaping the integrated outputsignal includes (i) at least one additional integrating amplifierconnected between the output of said input integrating amplifier and themodulator output means, for providing a second integrated output signal,(ii) means for weighting the integrated output signals of saidintegrating amplifiers so as to generate corresponding weightedintegrated output signals, (iii) means for summing said weightedintegrated output signals so as to generate a summed signal, and (iv)means for generating said intermediate digital signal as a function ofsaid summed signal.
 6. A scanner according to claim 4, wherein saidmodulator is a fifth order modulator.
 7. A CT scanner comprising: aplurality of detectors, each detector providing a relatively low levelanalog output current signal as a function of the amount of X-rayphotons detected by said detector during a predetermined sampling time;a delta-sigma A/D converter including a delta-sigma modulator having aloop and an integrating amplifier dispose within the loop of themodulator so that the input of the modulator can be connected to receivea relatively low level input current signal as a function of the amountof X-ray photons detected by a detector during a predetermined samplingtime so as to eliminate the need for a transimpedance preamplifier topreprocess the analog output current signal.
 8. A CT scanner accordingto claim 7, wherein the modulator is an integrated circuit, and theanalog output current signal of at least one detector is applied to theinput of the modulator.
 9. A CT scanner according to claim 7, whereinthe converter further includes noise shaping characteristics to improvefrequency response with minimal spillover.
 10. A CT scanner according toclaim 7, wherein the converter further includes a digital filter forfiltering the output of the modulator.
 11. A CT scanner according toclaim 10, wherein the digital filter includes a digital filter transferfunction optimized for CT-DAS applications.
 12. A CT scanner accordingto claim 11, wherein the digital filter transfer function includesfinite impulse response (FIR) filter characteristics.
 13. A method ofprocessing an analog output current signal of a detector of a CTscanner, comprising: applying a relatively low level input currentsignal as a function of the X-ray photons detected by a detector of a CTscanner during a predetermined sampling period to an input of adelta-sigma A/D converter including a delta-sigma modulator having aloop and an integrating amplifier dispose within the loop of themodulator so that the input of the modulator is connected to receive therelatively low level input current signal during a predeterminedsampling time so as to eliminate the need for a transimpedancepreamplifier to preprocess the analog output current signal of thedetector.
 14. A method according to claim 13, wherein the step ofapplying the relatively low level input current signal as a function ofthe X-ray photons includes applying the low level input current signalto the input of a delta-sigma modulator, wherein the modulator is formedas a part of an integrated circuit.
 15. A method according to claim 13,wherein the converter further includes noise shaping characteristics toimprove frequency response with minimal spillover.
 16. A methodaccording to claim 13, wherein the converter further includes a digitalfilter for filtering the output of the modulator.
 17. A method of claim16, wherein the digital filter includes a digital filter transferfunction optimized for CT-DAS applications.
 18. A method according toclaim 17, wherein the digital filter transfer function includes finiteimpulse response (FIR) filter characteristics.
 19. A method ofoversampling an analog information input current signal with anoversampling, delta-sigma A/D converter, and generating a digital signalrepresentative of the input current signal, comprising: receiving theanalog information input current signal at an input to the converter;generating an integrated output signal as a function of the analoginformation input current signal including: (1) noise shaping theintegrated output signal so as to provide an analog noise shaped signal;(2) converting the analog noise shaped signal to an intermediate digitalsignal as a function of an analog filtered signal at a sampling rate off₁; and (3) generating an analog feedback signal as a function of saidintermediate digital signal and applying the analog feedback signal tothe input of the modulator such that the integrated output signal is afunction of the analog information input current signal and the analogfeedback signal; and decimating the data rate of and filtering theintermediate digital signal so as to generate the digital output signalat a word rate f₂.
 20. A method of oversampling an analog current inputinformation signal with a delta-sigma A/D converter, comprising:receiving an analog information signal as a function of said analoginformation input current with a delta-sigma oversampling, A/Dconverter, comprising: (a) providing an intermediate digital signalrepresentative of said analog information input current, (b) generatingan integrated output signal as a function of the input current, (c)noise shaping the integrated output signal so as to provide an analognoise shaped signal; (d) converting the analog noise shaped signal tosaid intermediate digital signal as a function of the analog filteredsignal at a sampling rate of f₁; and (e) generating an analog feedbacksignal as a function of said intermediate digital signal and applyingsaid analog feedback signal to a modulator input; and decimating thedata rate of and for filtering said intermediate digital signal so as togenerate said digital output signal at a word rate f₂.
 21. A methodaccording to claim 20, wherein noise shaping the integrated outputsignal includes, providing a second integrated output signal, weightingthe first and second integrated output signals of said integratingamplifiers so as to generate corresponding weighted integrated outputsignals, summing said weighted integrated output signals so as togenerate a summed signal, and (iv) generating said intermediate digitalsignal as a function of said summed signal.